The leaves on a gardenia turn yellow when the air temperature is too cold or when the soil does not drain adequately. Yellow leaves also occur when the pH level of the soil is not between 5.0 and 6.5.
Gardenias are not hardy plants. Too much fertilizer burns the roots, restricting the intake of needed nutrients and causing the leaves to yellow. It is best to add fertilizers and nutrients to the soil about 1 foot away from the plant to keep them from being absorbed at full strength. An infestation of garden bugs is another cause of yellow leaves. Neem oil is a pesticide that repels the bugs but is not harmful to birds and other wildlife.